Micro-electro-mechanical systems (MEMS) involve the fabrication of small mechanical devices integrating sensors, actuators, mechanical elements and electronics on a silicon substrate at the micrometer scale. MEMS devices are manufactured through micromachining processes such as deposition, lithography and etching and are frequently used in the biomedical and electronics industries.
MEMS micropumps are designed to handle small amounts of liquids, on the order of several microliters to several milliliters per minute. These microfluidic devices are used in a number of technologies, including inkjet printers and particularly in the biomedical arts such as in electrophoresis systems, microdosage drug delivery systems, biosensors and automated lab-on-a-chip applications. MEMS micropumps remain a promising area of medical care technology.
MEMS micropumps can be generally classified into two groups: mechanical pumps with moving parts and non-mechanical pumps with no moving parts. Mechanical micropumps can be further differentiated by the mechanism in which they operate, including peristaltic, reciprocating and rotary pumps. These pumps are operated using a variety of actuation mechanisms such as pneumatic, thermopneumatic, electrostatic and piezoelectric principles.
In order to maximize pump efficiency, some micropumps employ the use of check valves. These check valves permit forward fluid flow during the drive cycle of the pump while minimizing or preventing reverse flow of the fluid during the priming cycle. Examples of this type of design are given in U.S. Pat. No. 6,179,856 and U.S. published patent application number 2005/0089415. Another piezoelectric actuation of a valved pump is described in U.S. Pat. No. 5,215,446. The inlet and outlet arrangements of this pump do not operate completely independently, limiting the pump to applications where complete isolation of the inlet and the outlet circuits is required.
Pumps requiring valves suffer from a number of drawbacks. Wear and fatigue cause a drop in performance and reliability. Check valves can also introduce a significant pressure loss that reduces pump performance when used to pump viscous working fluids. If particle-laden working fluids are involved, such as blood, there is a risk of the suspended particles clogging the valve.
In order to avoid these drawbacks, a variety of different valveless pump arrangements can be used. For example, U.S. Pat. No. 4,648,807 discloses a compact piezoelectric fluidic air supply pump. Using piezoelectric actuation, this double chamber pump vibrates a diaphragm to deliver an air supply. The pump is elongated in a direction parallel to the plane of the diaphragm. Inlet and outlet passageways are also elongated in this same direction, thus limiting the pump to a limited number of air supply applications, while compromising pump efficiency.
U.S. Pat. No. 6,203,291 discloses a displacement pump in which a diaphragm extends across perpendicularly oriented flow-constricting inlet and outlet chambers. The pump utilizes a single, rounded pumping chamber. U.S. Pat. Nos. 6,227,809, 6,910,869 and 5,876,187 also disclose pumps with a single circular pumping chamber, which limits the pump to those applications having less demanding requirements for a given allocation space.
Another type of valveless pump is disclosed in U.S. Pat. No. 6,179,584. The pump is configured in a silicon chip and a piezoelectric actuation drives one side of a single silicon membrane, thus limiting the pump to applications where lesser drive levels are needed.
U.S. Pat. No. 6,729,856 discloses an electrostatic pump with elastic restoring forces, and is operated so that fluids are passed through the pump while avoiding the electric field of the electrostatic actuator. Only a single pumping chamber of hemispherical shape is employed, and while being capable of operation in a valveless mode, practical valve operations may require the pump to meet greater throughput requirements.
U.S. published patent application 2006/0083639 discloses a micropump of PDMS material utilizing lead-in and lead-out nozzle structures connected to a single pumping chamber. The pump membrane is driven by a piezoelectric actuator, with a single piezoelectric disk located on one side of a membrane. This is an example of a valveless pump which includes a control element comprising a nozzle, instead of a valve. The control element can also comprise a diffuser element in place of the valve. In certain applications, nozzles and diffusers may be constructed according to different design principles, although for purposes of the present invention, the two are generally interchangeable.
One important feature of the control element is that its internal passageway changes in cross-sectional size as the length of the control element is traversed. Preferably, the change in cross-sectional size is continuous, and the direction of change is constant, although cylindrical sections could be introduced in some instances. That is, it is generally preferred that the control element is either outwardly flared or inwardly flared, and may have a frusto-pyramidal or frusto-conical shape, for example.
The control element, operates by providing a flow channel with a gradually expanding cross-section so that differential flow resistance is different in the forward and reverse flow directions. However, limitations are encountered in the above-mentioned micropump, since only a single pumping chamber is provided, with a diaphragm actuated on only one side by a single piezoelectric disk. Also, the inlet and outlet nozzle structures are oriented perpendicular to the plane of the diaphragm, limiting the pump to a number of specialized applications, and hampering efficiency.
The efficiency of known piezoelectric valveless micropumps is governed by fluid leakage losses (volumetric efficiency), frictional losses (mechanical efficiency) and imperfect pump construction (hydraulic efficiency). In addition to poor performance, inefficient pumps require a larger power source to drive the piezoelectric actuation mechanism, increasing costs and size. Thus, there is a need for an optimized piezoelectric valveless micropump with improved efficiency and reliability.